External regulator and DTC 41 - JeepForum.com
 
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post #1 of 10 Old 04-05-2020, 05:39 PM Thread Starter
couldabin
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External regulator and DTC 41

I have a '95 Grand Cherokee 4.0, and after years of fading (and somewhat variable) system voltage resolved that problem by installing an external voltage regulator. I now have rock-solid 14.3V when the car is running but of course the CEL is on, and DTC 41 is recorded.

Does anyone know how the PCM determines that it is disconnected from the alternator's field terminal? The PCM cycles the connection to ground in order to control the alternator voltage, so I thought that if the wire to the PCM was given a ground connection (through a 1K ohm resistor), it might think it was connected. It doesn't. I read somewhere that it checks to see that the alternator voltage drops to near 0V when the wire is ungrounded, and close to 12V when it is grounded but I don't see how that is possible, since the wire is the only connection between the PCM and the alternator. Is it possible that it watches for changes in system voltage that track the varying grounding duty cycle?

I would like to fool the PCM because I want the CEL to be lit only when there actually is an issue.

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post #2 of 10 Old 04-05-2020, 06:09 PM
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The PCM cycles the field control wire path to ground to control the field strength and thus the voltage output of the alternator -- it determines the alternator output based on the system voltage being supplied to the PCM at the B+ feed. Since the voltage doesn't change in response to the PCM's field control input now, it's going to throw a 41 fault code... not really much of anything you can do about that outside of restoring the internal VR function.

Keep in mind a steady 14.3v output isn't necessarily the correct one for all instances. The PCM will vary the charge rate/voltage output based on the state of charge of the battery and the temperature.

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post #3 of 10 Old 04-05-2020, 06:56 PM Thread Starter
couldabin
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Do you have any information on how the PCM determines whether there is sufficient change in B+ to decide that the field terminal is connected? Before I installed the external VR, the system voltage was wandering around 12.1 - 12.4 even though a code scanner showed the target voltage to be 14.2. I'm wondering whether the PCM was getting anything out of the requested change in voltage, but it still didn't set any DTCs. I checked the voltage on the field post today (with the external VR connected) and it was about 11.5V. I imagine that reflects the duty cycle that was being used at that moment. What do you think about supplying some stepped down voltage (using a potentiometer) on the field wire that goes back to the PCM?
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post #4 of 10 Old 04-05-2020, 09:05 PM
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I don't have the specifics on what the PCM is wanting to see, sorry. System voltage for a 12v should never be less than ~13.2v with the alternator/generator operating, it definitely should not be 12.1-12.4 seeing as how a fully charged battery should actually be at 12.6-12.8 volts. Were you getting these readings across the battery posts or on the cluster gauge? Assuming you're talking about the voltage supply to the field coming from the splice off the ASD and not the field control wire, your reading of 11.5v is also lower than what it should be. At the moment your readings make me think you have numerous voltage drops in your electrical system that need to be cleaned up and tightened, and probably a similar issue with your ground cables.

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post #5 of 10 Old 04-06-2020, 01:58 AM
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Pretty simple in theory.
PCM is seeing B+ voltage. The field wire to PCM is @ B+ voltage when the field is NOT grounded, when PCM commands alt to charge the wire is grounded the voltage drops* and this is the change the PCM uses to confirm the fields is 'on or off.' PCM Also expects to see change in B+ with field commanded.

* 3volts is dancing around in my old memory.

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post #6 of 10 Old 04-06-2020, 05:34 AM Thread Starter
couldabin
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@mukluk , the 11.5V was on the field terminal going to the PCM. The other post (the one from the ASD) was showing 14.3V. I have tried to find a bad ground but so far not found one. One of the things I did was pull the connection off the PCM and check resistance on the three ground pins -- all showed 0 ohms between that and the battery post. I'm not saying there isn't a wiring problem somewhere but haven't been able to locate one.

@jtec , do you think that feeding (B+ voltage minus 3V) to the PCM would be a possible workaround? I am pretty sure I could achieve that with a voltage divider connected to the feed from the ASD.
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post #7 of 10 Old 04-06-2020, 06:33 AM
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I gotta ask - you say the PCM is getting voltage on the field wire, is setting a code when remote reg takes command - what was reason for the bypass seams the PCM is getting the voltage...

GTG

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post #8 of 10 Old 04-06-2020, 08:43 AM Thread Starter
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Sorry, I wasn't very clear in my explanation. To install the external VR, I physically disconnected the three-wire harness (GND, B+ and field-to-PCM). I then re-connected the ground, and used the B+ on the harness as the source exciter voltage. So at this point I am re-using two of the three connections. The field-to-PCM is not connected to anything. When I said I found 11.5V on the field pin, I was measuring it at the alternator with the engine running, but it was being controlled by the external regulator. It wasn't going back to the PCM. I have mulled reconnecting the field-to-PCM wire, in which case I would have simply piggy-backed the external regulator on the existing setup. Any thoughts on that? Without knowing the construction of the external regulator, I wonder if the PCM might fry it with its grounding duty cycle.
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post #9 of 10 Old 04-06-2020, 09:07 AM
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sorry family issues will have me else where mentally. Sorry but wanted to finish my thought..

WHY did you disconnect the stock wiring and wire a separate regulator?

I doubt you could satisfy the PCM issue with a separate regulator, the PCM and add on would have different 'set points' the add would come on and off at a different time from the PCM logic, causing confusion on field circuit.

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post #10 of 10 Old 04-06-2020, 09:40 AM Thread Starter
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I went to an external VR because the PCM just wasn't doing the job. We have owned the Jeep for more than 20 years, and it now has 330K on it. Over the past few years the system voltage has become increasingly flaky -- almost never 14V while running, and too often less than 13V. I could not find anything wrong with the grounds or wiring. This is the third alternator we have put in it (the one I just took out doesn't seem to have anything wrong with it). The external VR brings system voltage to a much more appropriate level. I realize that ideally it would take into account battery temperature, but I think that a constant 14.2V is better than what I had.

I am considering trying this, and let me know if you have any thoughts on it: connect the field terminal to the original field-to-PCM wire, but place a 1K ohm resistor in-line. The external VR would still be controlling the alternator, but the connection allows the PCM to see the voltage on the field pin. Grounding by the PCM would essentially do nothing since only 0.014A would get through. Bad idea?
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